1. Field of the Invention
This invention relates to imparting a skin pass rolling operation to a grain oriented silicon steel strip subsequent to mechanical refinement of the magnetic domain spacing by patterns of scribe lines extending in a direction transversely of the strip.
2. Description of the Prior Art
Grain-oriented silicon steel is conventionally used in electrical applications, such as power transformers, distribution transformers, generators, and the like. The steel's ability to permit cyclic reversals of the applied magnetic field with only limited energy loss is a most important property. A reduction of this loss, which is termed "core loss", is highly desirable in the aforesiad electrical applications.
In the manufacture of grain-oriented silicon steel, it is known that the Goss secondary recrystallization texture, (110)[001]in terms of Miller's indices, results in improved magnetic properties, particularly permeability and core loss over non-oriented silicon steels. The Goss texture refers to the body-centered cubic lattice comprising the grain or crystal being oriented in the cube-on-edge position. The texture or grain orientation of this type has a cube edge parallel to the rolling direction and in the plane of rolling, with the (110) plane being in the sheet plane. As is well known, steels having this orientation are characterized by a relatively high permeability in the rolling direction and a relatively low permeability in a direction at right angles thereto.
In the manufacture of grain-oriented silicon steel, typical steps include providing a melt having on the order of 2-4.5% silicon; casting the melt; hot rolling; cold rolling the steel to final gauge typically of 7 or 9 mils, and up to 14 mils in one or more cold rolling stages, with intermediate annealing when two or more cold rollings are used; decarburizing the steel; applying a refractory oxide base coating, such as a magnesium oxide coating, to the steel; and final texture annealing the steel at elevated temperatures in order to produce the desired secondary recrystallization and purification treatment to remove impurities such as nitrogen and sulfur. The development of the cube-on-edge orientation is dependent upon the mechanism of secondary recrystallization wherein, during recrystallization, secondary cube-on-edge oriented grains are preferentially grown at the expense of primary grains having a different and undesirable orientation.
As used herein, "sheet" and "strip" are used interchangeably and mean the same unless otherwise specified.
It is also known that through the efforts of many prior art workers, cube-on-edge grain-oriented silicon steels generally fall into two basic categories: first, regular or conventional grain-oriented silicon steel; and second, high permeability, grain-oriented silicon steel. Regular, grain-oriented silicon steel is generally characterized by a permeability of less than 1870 at 10 Oersteds. High permeability, grain-oriented silicon steels are characterized by higher permeabilities which may be the result of composition changes alone or together with process changes. For example, high permeability silicon steels may contain nitrides, sulfides, selenides, and/or borides which contribute to the particles of the inhibition system which is essential to the secondary recrystallization process for the steel. Furthermore, such high permeability silicon steels generally undergo greater cold reduction to final gauge than regular grain oriented steels. A heavy final cold reduction on the order of greater than 80% is generally made in order to facilitate the high permeability grain orientation. While such higher permeability materials are desirable, such materials tend to produce larger magnetic domains than conventional material. Generally, larger domains are detrimental to core loss.
Once the steel obtains the grain-oriented texture, any subsequent cold rolling of the steel sheet is highly undesirable. It is well known that such rolling of the main body of the steel leads to unrecoverable deterioration in magnetic properties of the steel. The grain-oriented texture becomes disrupted as a result of such rolling.
It is known that one of the ways that domain size and thereby core loss values of electrical steels may be reduced occurs when the steel is subjected to any one of various practices designed to induce localized strains in the surface of the steel. Such practices may be generally referred to as "domain refining by scribing" and are performed after the final high temperature annealing operation. If the steel is scribed after the final texture annealing, then a localized stress state in the texture-annealed sheet is induced so that the domain wall spacing is reduced. These disturbances typically are relatively narrow, straight line patterns, or scribes, generally spaced at regular intervals. The scribe lines are substantially transverse to the rolling direction and typically are applied to only one side of the steel.
In fabricating electrical steels into transformers, the steel inevitably suffers some deterioration in core loss quality due to cutting, bending, and construction of cores during fabrication, all of which impart undesirable stresses in the material. During fabrication incidental to the production of stacked core transformers and, more particularly, power transformers in the United States, the deterioration in core loss quality due to fabrication is not so severe that a stress relief anneal (SRA), typically about 1475.degree. F. (801.degree. C.), is essential to restore properties. It is accordingly frequent practice not to apply this anneal. Thus, for such end uses, the need is for a flat, domain-refined silicon steel which will not necessarily be subjected to stress relief annealing. In other words, the scribed steel used for this purpose does not have to possess domain refinement which is resistant to the heating of a stress relief anneal.
However, during the fabrication incidental to the production of most distribution transformers in the United States, the steel strip is cut and subjected to various bending and shaping operations which produce more working stresses in the steel than in the case of power transformers. In such instances, it is necessary and conventional for manufacturers to stress relief anneal (SRA) the product to relieve such stresses. During stress relief annealing, it has been found that the beneficial effect on core loss resulting from some scribing techniques, such as mechanical and thermal scribing, are lost. For such end uses, it is required and desired that the product exhibit heat resistant domain refinement (HRDR) in order to retain the improvements in core loss values resulting from scribing.
In referring now to certain prior teaching, U.S. Pat. Nos. 4,533,409, issued Dec. 19, 1984 and 4,711,113, issued Dec. 8, 1987, disclose a method and apparatus for scribing a grain-oriented silicon steel to refine the grain structure by passing the cold strip through a roll pass defined by an anvil roll and scribing roll having a surface with a plurality of projections extending along and generally parallel to the roll axis. The anvil roll is typically constructed from a material that is relatively more elastic than the material from which the scribing roll is constructed. Preferably, the scribing roll is constructed from steel and the anvil roll is constructed from rubber. The process described in U.S. Pat. No. 4,711,113, may be performed before or after final texture annealing but the domain refinement achieved is not maintained through the usual stress relief annealing temperatures.
U.S. Pat. No. 4,742,706, issued May 10, 1988, discloses an apparatus for imparting strain to a moving steel sheet at linear spaced-apart, deformed regions. The apparatus includes a strain imparting roll having a plurality of projections as in the above described U.S. Pat. No. 4,711,113, except that the projections are formed on a spiral relative to the axes of rotation of the roll. The apparatus of the '706 patent also includes a press roll, a plurality of back-up rolls and a fluid pressure cylinder interconnected so as to control pressure against the press roll.
U.S. Pat. No. 4,770,720, issued Sep. 13, 1988 discloses a cold deformation technique wherein final texture annealed grain oriented silicon steel at as low as room temperature, and as high as from 50.degree. to 500.degree. C. (122.degree. to 932.degree. F.) is subjected to local loading, at a mean load of 90 to 220 kg/mm.sup.2 to (127,000 to 325,000 PSI) to form spaced apart grooves. The sheet must then be annealed at 750.degree. C. (1380.degree. F.) or more so that fine recrystallized grains are formed to divide the magnetic domains and improve core loss values which survive subsequent stress relief annealing.
In U.S. Pat. Nos. 5,080,326, issued Jan. 14, 1992 and 5,123,977, issued Jun. 23, 1992 and assigned to the same assignee of this patent application, a hot deformation technique is disclosed wherein the steel sheet is heated to a temperature in the range of 1000.degree. F. to 1400.degree. F. (540.degree. C. to 760.degree. C.) and while in this state it is locally hot deformed to facilitate the development of localized fine recrystallized grains in the vicinity of the areas of localized deformations to effect heat resistant domain refinement and core loss.
While the above prior attempts have, to different degrees, met the basic objectives to which they were addressed, they have created other technical and practical problems which the present invention is designed to overcome. One such problem is the stacking factor of the core assembly of the transformer. The stacking factor has reference to the efficiency of packing a stack of lamination with a maximum number of scribed sheets as compared to solid steel in a given cross section which are used to make up a transformer core assembly. Numerically, the term stacking factor is expressed as a percent ratio of a theoretical stacking height, i.e., calculated from weight, volume and density, to actual height under a given pressure. A high stacking factor permits more magnetic flux-carrying material in a given core volume. A 100% stacking factor is ideal although conventional flat electrical sheets yield a stacking factor on the order of 95%. Corrugations of the type developed as a result of mechanical scribing consisting of a multiplicity of closely spaced scribe lines traversing the strip are deleterious to the stacking factor although greatly beneficial to the refinement of magnetic domain wall spacing.
The stacking factor affects the capacity or power rating and size of the transformer and hence the ultimate use and cost. The stacking dimension is "enlarged" by the degree of penetration of the localized deformations cause by scribing and the non-uniformity in a linear direction of the deformations, (i.e. variation in the depth of the deformations). These two conditions of non-uniformity and excessive penetration of some of prior deformation techniques are also objectionable because they create problems in operation of the core-winding machine and gap patterns of the elements of the core and in the ease of moving and manipulating the scribed sheets during processing in the manufacturing of the transformers.